Learning Objectives
- Understand classification and nomenclature of amines
- Learn preparation methods and physical properties
- Study basicity of amines and factors affecting it
- Learn important reactions including diazotisation
- Distinguish between primary, secondary, and tertiary amines
Key Concepts
Classification
Primary (1°): R-NH₂ (one alkyl/aryl group). Secondary (2°): R₂NH. Tertiary (3°): R₃N.
Aliphatic amines: R groups are alkyl. Aromatic amines (aryl): -NH₂ attached to benzene ring (aniline: C₆H₅NH₂).
Preparation
- Alkylation of NH₃ (Hofmann): NH₃ + RX → RNH₂ + R₂NH + R₃N + R₄N⁺X⁻ (mixture of amines). Excess NH₃ favours 1°.
- Gabriel phthalimide synthesis: Phthalimide + KOH → potassium phthalimide + RX → N-alkylphthalimide → R-NH₂ (hydrolysis). Gives pure 1° amine only.
- Reduction of nitro compounds: R-NO₂ + 6[H] → R-NH₂ + 2H₂O. For aniline: C₆H₅NO₂ + Sn/HCl → C₆H₅NH₂.
- Reduction of nitriles: R-CN + 4[H] (LiAlH₄ or Na/EtOH) → R-CH₂-NH₂.
- Hofmann bromamide degradation: RCONH₂ + Br₂ + 4NaOH → R-NH₂ + Na₂CO₃ + 2NaBr + 2H₂O. Product has one less carbon.
Basicity of Amines
Amines are basic due to the lone pair on nitrogen.
In gas phase: 3° > 2° > 1° > NH₃ (inductive effect of alkyl groups dominates).
In aqueous solution: 2° > 1° > 3° > NH₃ (for small alkyl groups; solvation and steric effects compete with inductive effect).
Aromatic amines: Much weaker bases than aliphatic amines due to resonance delocalisation of lone pair into the ring.
Basicity order: (C₂H₅)₂NH > C₂H₅NH₂ > NH₃ > C₆H₅NH₂ > C₆H₅NHC₆H₅.
Effect of substituents on aniline: EDG (electron-donating, -CH₃, -OCH₃) increase basicity. EWG (electron-withdrawing, -NO₂, -CN) decrease basicity. p-nitroaniline is much less basic than aniline.
Chemical Reactions
Acylation: RNH₂ + CH₃COCl → RNHCOCH₃ + HCl (amide formation). 3° amines cannot be acylated.
Carbylamine (isocyanide) reaction: R-NH₂ + CHCl₃ + 3KOH → R-NC + 3KCl + 3H₂O. Foul smell of isocyanide. Test for 1° amines only.
Hinsberg's test: Distinguishes 1°, 2°, 3° amines using benzenesulphonyl chloride (C₆H₅SO₂Cl).
- 1°: Soluble in NaOH (sulphonamide with acidic H).
- 2°: Insoluble in NaOH (no acidic H).
- 3°: No reaction.
Diazotisation and Reactions of Diazonium Salts
Diazotisation: ArNH₂ + NaNO₂ + 2HCl → ArN₂⁺Cl⁻ + NaCl + 2H₂O (at 0-5°C).
Diazonium salt reactions (ArN₂⁺):
- Sandmeyer: ArN₂⁺ + CuCl → ArCl; + CuBr → ArBr; + CuCN → ArCN.
- Gattermann: ArN₂⁺ + Cu/HCl → ArCl (without Cu salt).
- Schiemann: ArN₂⁺ + HBF₄ → ArF + N₂ + BF₃.
- Replacement by -OH: ArN₂⁺ + H₂O (warm) → ArOH + N₂.
- Replacement by -H: ArN₂⁺ + H₃PO₂ → ArH + N₂.
- Coupling reaction: ArN₂⁺ + ArOH → Ar-N=N-Ar'OH (azo dye). ArN₂⁺ + ArNH₂ → Ar-N=N-Ar'NH₂. Used in dye manufacture.
Distinction Tests
- Carbylamine test: only 1° amines (foul smell).
- Hinsberg test: distinguishes all three.
- Nitrous acid (HNO₂) test: 1° aliphatic → N₂ gas + alcohol; 1° aromatic → diazonium salt (0-5°C); 2° → N-nitrosoamine (yellow oily liquid); 3° aliphatic → no visible reaction; 3° aromatic → p-nitrosoamine.
Summary
Amines are nitrogen-containing organic bases classified as 1°, 2°, or 3°. They are prepared by alkylation, reduction of nitro compounds, or Gabriel synthesis. Basicity depends on electronic effects, solvation, and steric factors. Important reactions include acylation, carbylamine reaction (1° test), and Hinsberg test. Aromatic amines form diazonium salts at 0-5°C, which are versatile intermediates for Sandmeyer reactions and azo dye synthesis.
Important Terms
- Diazotisation: Formation of diazonium salt from aromatic amine
- Sandmeyer Reaction: Replacement of N₂⁺ by Cl/Br/CN using Cu salts
- Coupling Reaction: Diazonium salt + phenol/amine → azo compound (dye)
- Carbylamine Reaction: Test for primary amines (foul-smelling isocyanide)
- Gabriel Synthesis: Preparation of pure primary amines
- Hofmann Degradation: RCONH₂ → RNH₂ (one carbon less)
Quick Revision
- Basicity (aqueous): 2° > 1° > 3° > NH₃ >> ArNH₂
- Gabriel synthesis: only 1° amines (not ArNH₂)
- Hofmann degradation: RCONH₂ + Br₂/NaOH → RNH₂ (one C less)
- Carbylamine: 1° only; Hinsberg: distinguishes all three
- Diazotisation at 0-5°C; Sandmeyer: CuX for ArX
- Coupling with phenol: azo dye (coloured compound)